1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive vehicles, gas pumps and cogeneration systems, and more particularly to a spark plug with a ground electrode having installed therein an Ir-alloy chip.
2. Background Art
Japanese Patent First Publication No. 8-298178 discloses a spark plug equipped with an Ir-alloy chip. The spark plug includes a center electrode and a ground electrode. The center electrode is disposed within a metal shell through a porcelain insulator and has a tip exposed outside an end of the metal shell. The ground electrode is joined to the end of the metal shell and has a spark discharging surface formed on an end thereof which defines an air gap (also called a spark plug gap) between itself and the tip of the center electrode. The Ir-alloy chip is installed on the spark discharging surface of the ground electrode for producing a sequential of sparks between itself and the end of the center electrode.
When the spark plug is used in an internal combustion engine, the Ir-alloy chip is subjected to intense heat. The heat principally dissipates from the Ir-alloy chip to the ground electrode and to the metal shell and the atmosphere. The Ir-alloy chip is bonded to the surface of the ground electrode through a corrosion resisting non-noble metallic member. Specifically, the whole of the Ir-alloy chip lies over the surface of the ground electrode. This structure, therefore, arrests the transmission of heat from the Ir-alloy chip to the ground electrode, so that the Ir-alloy chip is exposed to intense heat for a long time, resulting in acceleration of oxidation and wear of the Ir-alloy chip.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a spark plug with an Ir-alloy chip joined to a ground electrode which is designed to provide a desired amount of heat dissipation from the Ir-alloy chip.
According to one aspect of the invention, there is provided a spark plug which may be employed in automotive vehicles, gas pumps and cogeneration systems. The spark plug comprises: (a) metal shell; (b) a center electrode retained within the metal shell to be insulated from the metal shell; (c) a ground electrode joined to the metal plug, the ground electrode having a center electrode-facing surface opposed to a tip of the center electrode through a spark plug gap; and (d) an Ir-alloy chip working to produce a spark between itself and the tip of the center electrode, the Ir-alloy chip being embedded in the center electrode-facing surface of the ground electrode with a portion thereof exposed outside the center electrode-facing surface of the ground electrode.
In the preferred mode of the invention, the Ir-alloy chip other than the exposed portion thereof is installed inside the ground electrode.
The Ir-alloy chip may alternatively have at least one surface which lies flush with a side surface of the ground electrode continuing from a peripheral edge of the center electrode-facing surface.
The exposed portion of the Ir-alloy chip projects from the center electrode-facing surface of the ground electrode toward the center electrode.
The Ir-alloy chip is joined to the ground electrode through at least one fused portion in which materials of the Ir-alloy chip and the ground electrode are melted together. The fused portion may be formed by laser welding.
The shortest distance between the fused portion and the center electrode is more than or equal to the sum of an interval between the tip of the center electrode and the Ir-alloy chip through the spark plug gap and 0.3 mm.
The ground electrode has a recess formed in the center electrode-facing surface. The Ir-alloy chip is fitted within the recess. The fused portion extends continuously from an outer side wall of the ground electrode inside the Ir-alloy chip through an outer side wall of the Ir-alloy chip.
The ground electrode has a second surface opposed to the center electrode-facing surface. The tip of the fused portion lies within the Ir-alloy chip closer to the center electrode-facing surface than the second surface. The distance between the tip of the fused portion and the bottom of the Ir-alloy chip lying inside the ground electrode is greater than or equal to 0.1 mm.
The length of a part of the fused portion lying within the Ir-alloy chip is greater than or equal to 0.2 mm.
The distance between the tip of the fused portion and the center electrode-facing surface of the ground electrode is greater than or equal to 0.2 mm.
The distance between the outer side wall of the Ir-alloy chip and the outer side wall of the ground electrode is greater than or equal to 0.25 mm.
The fused portion may lie close to a joint of the ground electrode and the metal shell from a center line of the Ir-alloy chip extending toward the center electrode through the spark plug gap.
The distance between an end of the exposed portion of the Ir-alloy chip oriented toward the center electrode and the center electrode-facing surface of the ground electrode lies within a range of 0.1 mm to 1.0 mm.
The Ir-alloy chip is made from material containing a main component of Ir (Iridium) and an additive of at least one of Rh (rhodium), Pt (platinum), Ru (ruthenium), Pd (palladium), and W (tungsten). The Ir-alloy chip may contain 70 to 99 Wt % of Ir.
The Ir-alloy chip may be joined to the ground electrode through a plurality of fused portions in which materials of the Ir-alloy chip and the ground electrode are melted together. In this case, at least one of the fused portion lies preferably close to a joint of the ground electrode and the metal shell from a center line of the Ir-alloy chip extending toward the center electrode through the spark plug gap for increasing the degree of joining of the Ir-alloy chip to the ground electrode.